Previously, Automated Test Equipment have attempted to measure high frequency signals, but the accurate measurement of these high frequency signals has proven difficult. The results of the measurements are often unsatisfactory. One reason for the difficulty is related to the parasitic effects of the hardware fixturing of the ATE equipment. Stray capacitance and inductance from the fixturing such as wafer probes, handler contactors and device sockets may have a detrimental effect on the high frequency electrical signals to be measured. Exemplary, the settling time of a palette digital to analog converter (DAC) could be difficult to accurately measure through a long wire since the inductance of this long wire would adversely affect the signal to be measured.
Matched impedance terminated coaxial cables have been used successfully in these ATE test fixtures to reduce the inductive and capacitive effects of the signal transmission path of the cables. A properly terminated coaxial cable appears as a purely resistive load to the device under test (DUT). Such terminated cables can be successfully used to measure a signal if the device under test (DUT) is capable of driving the termination resistance without significantly affecting the signal under test. Typical ATE coaxial cables may require a termination resistance of either fifty or ninety Ohms and as such require the DUT to drive a fifty or ninety Ohm resistive load. Many devices under test do not have such low impedance drive capability and as a consequence the ATE equipment cannot easily be used to measure high frequency electrical signals.
Measurements of these high frequency signals are also affected adversely by the limited speed of the A/D converters of the prior art. These prior art converters are adapted to operate at higher sampling rates which disadvantageously limits their voltage resolution. Exemplary, if a converter operates at a required sampling rate of 20 MHZ, the digitizer of the converter may have a resolution of only 12 bits instead of the preferred 16 to 24 bits common to lower frequency converters.
In certain prior art high speed digitizers, the stray inductance/capacitance and sampling resolution problems have been reduced by positioning a very high speed strobed comparator near the device under test (DUT). This comparator is used as a front-end to an undersampling successive approximation routine (SAR) A/D converter. Positioning the comparator near to the DUT may reduce the effects of the stray capacitance and inductance on the signal to be tested. However, there is a physical limit to how close the comparator can be positioned near to the DUT. When the comparator is positioned on an integrated circuit chip (IC) separated from the circuit under test, this separation of the two IC's imposes limits to how close the devices may be positioned together. As a consequence of this limitation, the separation prevents this elimination of additional stray capacitance and inductance.